Suddenly I cannot see Time Capsule and my Networked Time Capsule unless I connect to my router on wifi

Did something get an upgrade or a patch.. firmware on main router or TC or computers?

Have you powered everything off.. then restart in correct order.

Main modem.. wait 2min

Main router.. (if different to modem) wait 2min

TC1.. if TC2 is dependent on TC1.. start them separately.. otherwise together.. wait 2min.

Does it now work?

No luck ???

You will need to give us a lot more info on your network.

What is the make and model of your modem? Is it also your main router?

How are the Time Capsules actually configured?

A screenshot from the Mac that works or using wifi is also fine.. showing the summary page of each TC in the airport utility.

I would also like to see your network preferences info from wireless when working.. and with wireless off and just using ethernet.

Please show us the DNS and domain stuff in the advanced section.

This is important. As it works with wireless but fails on ethernet it can be getting different values.

Make sure your IPv6 is set to link-local only unless you are actually running IPv6 full network.

Post the screenshots from the wifi as well.

Nothing here is going to cause security issues.. I posted mine so you can see that it is all private addressing.

Also when you turn off wireless on the computer that is having issues.. try using network utility and ping the TC by name and IP. Tell us your results.

Is the high sierra computer running any anti-virus software or extra security or adware. All of them will cause issues.. and should be turned off.. and best if they are uninstalled.

First thing to do is change the IPv6 setting to link-local only.

Reboot the computer and tell me what happens.

As you can tell the Automatic setting can get the setting right or wrong. At some point Mac 2 is likely going to suffer the same problem.. at which time you can change it to link-local only. For now just leave it on auto.. but it is no problem if you would prefer to swap it to link-local only.

I have a setup where I modify things all the time to answer questions here.. the result is the computer goes back to auto whenever I do a reconfigure and works fine most of the time. How Apple can get something so simple wrong in the latest OS is amazing but I think is partly due to your setup with the Netgear running as the main router. This setting has been a problem for a long time.

There is a bunch of other stuff you can do if you have more issues. Just post back and we can look at some of the other settings.

Make sure that your Mac that is using an Ethernet connection, that that connection is at the top of the list of Network connection in System Preferences > Network, as in the image below: